Download The Build-Up of Bimetallic Transition Metal Clusters

The Build-Up of Bimetallic Transition
Metal Clusters
By Paul R. Raithby
Department of Chemistry, University of‘Cambridge, England
The synthesis and reaction chemistry of high nuclearity transition metal
carbonyl clusters is briejly reviewed, and new synthetic strategies leading to the
“rational” synthesis of bimetallic clusters containing metal cores of over 1 nm
i n dimension are described. The solid state structures of a number of usmiuml
mercury, osmiumlgold and rutheniumlcopper bimetallic clusters are discussed
with regard to the nature of their formation, und of their bonding and redox
properties. Suggestions are made as to how the synthetic strategies can be adapted
to prepare bimetallic clusters of industrially useful combinations of metals.
Recent work showing that bimetallic nunuparticles prepared frum clusters are
catalytically active when anchored inside mesoporous silica is also discussed.
Transition metal carbonyl cluster chemistry
has been an important and developing topic
of research in organometallic chemistry for the
last three decades (1). One of the main appeals
of clusters is that they lie at the interface between
‘‘conventional” organometallic chemistry and
the chemistry of colloids and of the bulk metal.
Figure 1 illustrates the progression in particle
size from a single atom through clusters, with
metal core sizes of around 1 nm; nanoparticles,
with sizes up to 100 nm; leading into the colloid regime; and then on to the bulk metal.
Indeed, at what size (number of metal atoms)
does a metal cluster stop behaving like an
organometallic complex, with bonding properties that can be described in terms of discrete
molecular orbitals, and take on metallic properties, where the bonding can be described in
terms of band structure? There is no immediate answer to this question, but there is a clear
progression towards the clusters taking on metal-
Single
metal
atom
Particle diameter
* .
;.;.: ..
Cluster
roi
- .*...
Nanoparticle
-lo2i
Platinum Metals Rev., 1998, 42, (4), 146-157
Colloid
4 0 3 i
lic properties as the nuclearity increases,
although different sizes of cluster exhibit different types of metal-like properties under
different conditions (2).
One of the main thrusts of cluster chemistry
at Cambridge has been to prepare ever larger
transition metal clusters and to investigate their
physical and chemical properties. A range of
clusters containing more than ten metal atoms
has now been prepared and crystallographically
characterised ( 3 ) and examples in which the
metal atoms “condense” to form structures corresponding to the hexagonal, cubic and bodycentred cubic packings found in bulk metal have
been observed, as well as other clusters, such as
[Pt19(C0)22]4(4), which exhibit five-fold symmetry packing.
The diameters of the metal cores in the largest
of these clusters, such as [Ni,,Pt,(CO)4aH,,]”~
(n = 5, 4) are of the order of 2 nm (5). Even
larger clusters containing copper and selenium
4
Bulk metal
)lo%
Fig. 1 The progression in particle
size from a single metal atom to
the bulk metal
146
have been prepared, and the largest of these to
have been crystallographicallycharacterised is
[ C U ~ ~ ~ S ~ in
~ which
~ ( P the
P ~selenium
~ ) ~ ~ atoms
]
exhibit “ABA” stacking and the copper atoms
occupy interstitial sites (6). There are also
reports of transition metal clusters, for instance
those containing Auss(7) and Rhss( 8 ) , Pt,09(9)
and Pd,,, (10) units, and a series of palladium
clusters containing up to 2000 metal atoms ( I I),
which have not yet been crystallographically
characterised, but which must have dimensions
of the order of 4 nm.
Several research groups have proposed that
clusters can act as good building blocks in
nanoscale architecture and thus will find application in the fabrication of single electron devices
(12 ) . Small metal particles and other transition
metal clusters have also been clearly shown to
form densely packed monolayers on electron
microscope grids when they are ligated by
organic surfactant molecules (13).
From the viewpoint of catalysis, metal clusters can be considered as fragments of a metal
surface surrounded by a layer of “adsorbed” ligand molecules. Even the largest osmium cluster carbonyl so far characterised, [Os,,(CO),]’~,
where the metal framework is approximately
0.9 x 0.9 x 0.9 nm in size, contains only surface
atoms, with each osmium atom being bonded
to at least one carbonyl ligand, see Figure 2,
(14).
However, can clusters be regarded as good
models of metal surfaces in heterogeneous catalytic reactions? The “cluster/surface” analogy
was pointed out quite early in the development
of cluster chemistry (15), and ever since then
clusters have been used as models for catalytic
systems (1 6).
Certainly, organic molecules bond to catalytically active metal surfaces in the same way as
to metal clusters, and analysis of cluster systems
is easier because they can be subjected to the
full range of solution spectroscopic techniques,
such as IR and multinuclear NMR spectroscopies, mass spectrometry and, in the solid
state, single crystal X-ray crystallography,
whereas analysing the bonding modes of
co-ordinated molecules on a metal surface
Platinum Metals Rev., 1998, 42, (4)
Fig. 2
The metal core structure of
showing the four triangular
faces of the Osmtetrahedron
[Os,(CO),]*-
under catalytic conditions is rather more
challenging. However, for the majority of cluster models, for example [Os,,(CO),,]’- (14), only
“surface” atoms are present, and since the
sub-layers of the bulk metal influence the chemistry of the surface atoms on a catalytic surface,
some aspects of the model are not valid, so the
“analogy” should be treated with caution.
Therefore, the simple answer to the question
of whether clusters are good models for
heterogeneous catalysts would be “no”.
“Rational” Synthesis of High
Nuclearity Mixed-Metal Clusters
With a view to preparing precursor materials
that could have applications in catalysis and
in
nanoparticle technology, the “ c l ~ ~ t egroup
r”
Cambridge has been developing strategies for
synthesising high nuclearity mixed-metal clusters containing ten or more metal atoms in their
cluster cores. Even the smallest of these clusters should have a core diameter in excess of 0.5
nm and have the advantage, unlike bimetallic
particles prepared by other routes, that the exact
ratio of the two metallic elements is known.
As much of the early cluster synthesis work
147
I
Fig. 3 The formation of the “spiked”-triangular cluster [Os,H,(CO),,] from the reaction of the
activated cluster [OS~(CO)~~(M~CN)~]
with [OsHZ(CO)d]
involved pyrolysis or thermolysis techniques and
resulted in a range of products, all in low yields,
fiom a single reaction, it has been necessary to
develop synthetic strategies where one target
cluster molecule can be obtained in good yield
(17).
In order to achieve this, two fairly straightforward synthetic routes are available, given that
the starting materials for the production of high
nuclearity clusters are usually low nuclearity
carbonyl clusters.
The first route, illustrated in Figure 3 by the
formation of [ O S ~ H ~ ( C Oinvolves
) ~ ~ ] , the activation of the binary carbonyl [Os,(CO),,] with
Me,NO, in the presence of MeCN and results
in the oxidation of carbonyl ligands to carbon
dioxide and the occupation of the vacant coordination sites with labile MeCN ligands. The
subsequent addition of the neutral mononuclear
complex [OSH,(CO)~]displaces the MeCN
groups and affords the “spiked”-triangular clus(18). In this method the
ter [OS,H~(CO),~]
cluster nuclearity is increased by one, by the
reaction of a neutral, activated, low nuclearity
M = Ru, 0 5
cluster with a neutral monometal complex.
The second route involves the ionic coupling
reaction between a carbonyl cluster anion and
a monometal cationic species, again to increase
the cluster nuclearity by one. In Figure 4, the
tetranuclear osmium cluster [Os,H,(CO) ,,I is
initially reduced with WPh,CO to form the
dianion [Os,H,(CO) ,,I ‘-, and then treated
immediately with the labile cation [M(q6C6H6)(MeCN),I2+
(M = Ru, 0 s ) to form the
pentanuclear, neutral cluster [Os&W,(CO),,(q6C6H6)](1 9). By choosing appropriate cation
and anion charges and ratios, the cluster nuclearity can be increased by two units, as in the
reaction of [Os,(CO),,]’~with two equivalents
of [Ru(q5-CsH5)(MeCN),]’to give the pentanuclear cluster [Os3Ru,(CO),,(q’-C,H,),] (20).
This latter ionic coupling route has been particularly successful, and has been extensively
exploited to prepare a wide range of higher
nuclearity clusters containing carbocyclic
ligand groups (21).
The method has also been used by a number
of research teams for synthesisingmixed-metal
a
Insertion
NCMe
..!. \
(Oc)2
[H,Os,CCO),]
(C0)3
H ,,OS,,M(CO)~I(C~H~)
(M = Ru, 0 s ) from the coupling of
Fig. 4 The synthesis of [OsdMH,(CO),,(~6-C6H6)]
[OS~H~(CO
with
) ~ ~[M(t16-C6H6)(MeCN),]2+
]~~
(M = Ru, 0 s )
Platinum Metals Rev., 1998,42, (4)
148
%
P
b
Fig. 5 The structure of the “raft” cluster
[{Os,(CO)rlHg}s] showing the linking ofthree
Os, triangles to the central Hg3 triangle
clusters containing the coinage metals by the
reaction of carbonyl cluster anions with cationic
copper, silver and gold complexes (22).
Osmium-MercuryClusters
So far, our most extensive series of studies into
the formation of mixed-metal clusters containing ten or more metal atoms have also involved
the reaction of a range of ruthenium and osmium
cluster carbonyl anions with late transition metal
cations, such as [ H a ] ’ (X = C1, CF,), [AuPR,]’
(R = alkyl, aryl) and [CU(NCM~)~]’.
The first
indication that cluster build-up could occur to
produce higher nuclearity clusters came from
the metathesis reaction of [OS,H(CO)~~]~
with
mercury(I1) salts. The product of the reaction
was the extremely photolabile, dodecanuclear
‘‘rafl” cluster [ { Os,(CO) IHg},] shown in Figure
5 (23). It is significant that the three mercury
atoms form a central triangle with Hg-Hg
distances in the range 3.08-3.12 A. The
“OS,(CO)~~”
fragments bridge this central triangle, with each “Os(CO),” group forming
bonds to two mercury atoms (0s-Hg in the
range 2.71-2.76 A); the co-ordinated “Os(CO),”
groups each form one bond to a mercury atom
(0s-Hg in the range 2.98-3.05 A). This cluster can be viewed as a model for a bimetallic
surface, and while being only one layer thick
Platinum Metals Rev., 1998,42, (4)
has a core diameter in the surface plane of
approximately 1.2 nm (12 A).
This coupling process, more correctly called
redox condensation, resulting in cluster buildup, can also be achieved between the carbidostabilised, decanuclear cluster anion
[OsloC(CO)z4]z~
and [Hg(O,SCF,),] to afford
,Figure 6. Here
the anion [ { OsloC(CO)z4}zHg]2
the mercury atom links the two decanuclear
clusters by bridging an edge of each Oslounit,
forming a cluster containing 2 1 metal atoms
(24). In this reaction, the appropriate choice of
the mercury(I1) salt, [Hg(O,SCF,),], is the key
to the formation of the higher nuclearity mixedmetal cluster. This is partly because the decaosmium dianion precursor has low nucleophilicity (the negative charge being delocalised
over the ten metal centres) and because there is
an increased propensity for the reaction to
follow alternative pathways involving partial
degradation or rearrangement of the metal
L
b
Fig. 6 The molecular structure of the mercury
linked cluster dianion [{Os,,C(CO),,},Hg]*~
149
0
0
0
4 P L P 4
The Hg-Hg bonds in the Hg, dianion are an
average 2.927 A long, but in the HgZdianion
[Os,,Hg2C,(CO)42]2~,
which has also been crystallographically characterised and found to have
a pair of mercury atoms linking the two Os,
units, the Hg-Hg bond is significantly shorter
at 2.745 A long.
Osmium-Gold Clusters
Fig. 7 The structure of the dianion
[MisHgX~(Co)u]’-
framework. Again, the metal core in this cluster
is somewhat asymmetric but has a maximum
dimension of approximately 1.6 nm.
When a similar reaction occurs between
[M,,C(CO),,]’~(M = Os, Ru) and the mercury
salt [Hg(O,CCF,),], a different type of product,
[M18Hg3CZ(CO)IZ]2~,
is obtained (24, 25),
although the metal core still contains 21 metal
atoms. This also contains a central Hg, triangle, Figure 7, linking two “M9C(CO)ZI”
units,
derived from the framework of the tetracapped
octahedral starting materials [MI&(CO)zr]2by the loss of an “Os(CO),” vertex. As in
,I it is of interest that the mer[{OS,(CO),~H~}
cury atoms have linked together between two
osmiumhthenium cores forming, in metallurgical terms, a mercury domain. Another
fascinating feature of the cluster dianion
[ O S , , H ~ , C ~ ( C O ) is
~ ~that
] ~ - it undergoes
reversible photochemical and redox extrusion
of mercury atoms to give a complete series of
high nuclearity clusters with general formula
[Os,sHg,Cz(CO),2]”~
(n = 1-3, m = 1-4) (26).
Platinum Metals Rev., 1998, 42, (4)
A wide range of reactions which form high
nuclearity osmium-gold clusters has been carried out (27), but this discussion will be
restricted to two key syntheses which yield
important information about the build-up
processes in high nuclearity clusters. The first
reaction is that between [OS,~C(CO),,]~~
and the
polygold cation [(AuPR,),O]+(PR, = PCy,,
PPh,, PMeJ’h) which affords the fourteen atom
cluster [OsloC(CO) ,,Au(AuPR,) ,] (28). The
structure of the PCy, derivative has been characterised crystallographically and is shown in
Figure 8. The tetracapped octahedral geometry of the parent Os,, dianion is retained, and
the four gold atoms form a tetrahedral cluster
which is linked to the osmium core via one gold
atom that bridges an 0 s - 0 s edge of one of the
Q
I)
\8P
Fig. 8
The
molecular
structure
of
[OS~~C(CO),~U(A~PC~~):~]
150
Fig. 9 (left) The molecular structure of the osmium-goldcomplex [Osla(CO),,(AuPPhzMe),]
complex when it is viewed from
Fig. 10 (right) The structure of the [OS,~(CO)~,(AUPP~~M~),]
one end of the cluster. The novel tubular nature of the metal cnre composed of osmium atoms is
clearly visible
tetrahedral caps. The mean Au-Au distance to indicate a bonding interaction, while the truns
within the Au, tetrahedron is only 2.7 1 A, which axial osmium atoms [Os(2)...Os(3a) and
suggests that the Au-Au bonds are relatively Os(3)...Os(2a)l have moved closer together than
strong. As also observed for the osmium-mer- would be expected in an octahedron, to an avercury clusters, the gold atoms have a tendency age distance of 3.3 A. The pairs of gold atoms
to “cluster” together to form a domain, and are separated by 4.43 A, and the length of the
do not become incorporated into the osmium tube including the gold phosphine groups
atom framework.
exceeds 1 nm. In order to support this type of
In the second key reaction the non-carbido geometry the metal bonding must be delocalised
decaosmium cluster dianion [0~10(c0)26]’~
is in character.
further reduced with KPh,CO, presumably to
give a tetra-anion, that is treated in situ with Ruthenium-Copper Clusters
Of all the cations discussed above,
[AuPPh,Me]+ to give a new type of high
[Cu(NCMe),]’
is the most versatile for use in
nuclearity cluster [Oslo(CO),,(AuPPh2Me),],
see Figure 9 (29).
cluster build-up reactions. It has been used in
At first sight, from the structure of this four- combination with a number of ruthenium clusteen atom cluster, it appears that the four ter anions to produce a range of novel, high
[AuPPh,Me]+cations merely cap the four end nuclearity, mixed copper-ruthenium clusters.
faces of a bioctahedral osmium core, but the For example, in dichloromethane, the
view looking from one end of the cluster to the reaction of the octahedral ruthenium anion
other, Figure 10, shows that the octahedra are [ R U ~ ( ~ ~ - H ) ( C Owith
) , ~ ] ~an excess of
distorted and that a novel tubular structure has [Cu(NCMe),]’ affords the dianionic cluster
formed. The 0 s - 0 s equatorial edges [Os(4)- [{Ru,H(C0)12)2Cu7C1,]2~,
see Figure 11 (30).
Os(5a), Os(1)-Os(la), Os(5)-0s(4a)] have The fifteen-atom cluster core contains two
expanded to lie in the range 3.29-3.32 A, a dis- Ru, tetrahedra linked through a Cu7unit which
tance that is significantly longer than is judged may be described as two fused square-based
Platinum Metals Rev., 1998, 42, (4)
151
Fig. 11 The structure of the
[{RulH(CO)lr}rCu,Cld]2~
dianion
showing the central Cu: unit
- v
CIO)
pyramids sharing a common triangular face.
Three chloro ligands each symmetrically bridge
pairs of copper atoms, while the remaining copper atom, Cu(5), forms no bonds with ligands
but has eight metal contacts. Thus, the immediate environment around Cu(5) is similar to
that in metallic copper and, overall, again it is
seen that the element with the formal d'O electron configuration has formed the central
domain, and the transition metal units are fused
to its periphery.
The product of this reaction is very sensitive
to the nature of the solvent used. In the
presence of [(Ph,P),N]Cl in MeCN, when
[Ru,H(CO),J is treated with a large excess
of [Cu(NCMe),]+, a different product,
[{RU,H(CO),,),CU~C~,]~,
is obtained in good
yield, see Figure 12 (30). In this case degrada-
O(21I
Platinum Metals Rev., 1998, 42, (4)
tion of the original Ru, octahedron has not
occurred, and the two octahedra are linked
through two Cu, tetrahedra which share a common edge, generating two butterfly arrangements which bond to the ruthenium units.
Overall, the eighteen-atom cluster can be viewed
as a linear condensation of four octahedra. Two
edges of the Cu, unit are symmetrically bridged
by chloride ions.
For both reactions, the presence of chloride
ions is apparently necessary, even if, in the first
case, they are abstracted from the solvent.
However, by simply altering the solvent, from
dichloromethane to [(Ph,P),N] C1 in MeCN,
good yields of high nuclearity clusters with ruthenium:copper ratios of 8:7 (approximately 1: 1)
and 2:1, respectively, are obtained from a room
temperature reaction.
Fig. 12 The structure of the
[ {Ru,H(CO)17}2C~CI,]2~
dianion
152
Fig. 13 The core geometry in
the [ {R~~OHZ(CO)~~}~CU,CI~]~
dianion
This synthetic methodology can be expanded nuclearity cluster, was obtained in quantitative
further. The reaction of the decanuclear dian- yield, see Figure 14 (32). The two Ru, octaheion [RU,,H,(CO)~~]~~
with excess [CU(NCM~)~]+,dra are linked by a rectangular planar arrangein dichloromethane, in the presence of chloride ment of four copper atoms, opposite edges of
ions, affords the twenty-six-atom cluster which are bridged by chloride ions. Thus, with
[ {RuloH,(CO),,}2C~~ClZ]~~
with a 70 per cent chloride ions present in the starting material the
yield and a ruthenium:copper ratio of 10:3 (31). cluster build-up is not so efficient, resulting in
Here, the two Rule units are fused on either side a sixteen-atom cluster with a ruthenium:copper
of a Cu, unit which adopts the same geometry ratio of 3: 1, but the yields of the product are
as that found in [ { Ru,H(CO) !,} ,CU,CI,]~- improved.
In all of the copper-ruthenium clusters inves(Figure 12). The overall core geometry can be
described as six fused octahedra with an addi- tigated, the copper atoms condense to form a
tional ruthenium atom at each end capping a central domain and the ruthenium cluster units
butterfly face to form a trigonal bipyramid, condense around the periphery to produce nanoFigure 13; this metal framework is over 1.6 sized particles based on the fusion of octahedral
or tetrahedral units, just as is observed in other
nm long.
In order to investigate the role of the high nuclearity clusters and in close packed metchloride ions in these reactions, the carbido als (3). The advantage of the strategy employed
dianion [ R U ~ C ( C O ) ~ ~was
] * - treated with in the synthesis of these copper-ruthenium
CuCl, instead of the [Cu(NCMe)J+ cation, clusters is that by carefully controlling the reacanother high tion conditions, nanosized particles with
and [ {Ru,C(CO),,)2Cu,Cl,]2~,
Fig. 14 The structure
of
the
dianion
[{ h C ( C0)w) iC~,Clz]
*
Platinum Metals Rev., 1998, 42, (4)
153
Fig. 15 The network of reversible redox
transformations of [Os,,Hg,,C,(CO),,]”’(11 = 1-3, m = M),Fr = ferroeene
the species [OS,,H~,C~(CO)~,]”’
(n = 1-3, m =
1-4) (26) have been characterised, and it is not
possible to assign realistic oxidation states for
the individual mercury atoms in these species.
For example, in [OS~,H~,C~(CO),,]~-,
in order
to balance the formal -4 charge from each
“OS,C(CO)~,”fragment, each mercury atom
would have to be assigned an unrealistic oxidation state of +3. The network of reversible
redox transformations shown in Figure 15 confirms that this series of clusters can act as “electron sinks” (26) with a description of the bonding within the metal core best described in terms
of delocalised molecular orbitals.
Clusters as Nanoparticle Precursors
For the chemistry outlined above it is clear
that
it is now possible to prepare a wide variparticular ruthenium:copper ratios can be
ety
of
bimetallic, nanosized clusters in good yield
obtained in high yields. This control has not
with
specific,
known ratios of the two metals.
been evident in previous studies (3).
In principle, this strategy can be applied without difficulty to other combinations of transiSome Electrochemical
tion metal atoms, such as palladium-rhodium
Considerations
If higher nuclearity bimetallic clusters become and platinum-rhodium, and high nuclearity
“metallic” in character they would be expected clusters could be prepared. However, would
to undergo extensive redox chemistry, with the these systems have real uses with industrial applicluster cores behaving as “electron sinks”. For cations?
It has long been established that metal clusall of the copper-ruthenium clusters described
above it is possible to assign a formal oxidation ters can be anchored to oxide surfaces such as
state of + 1 to each of the copper atoms in the silica and alumina, and even encapsulated in
structures, despite the view that the bonding zeolite cages; after heating the residual metal
must be delocalised over the metal core. particles have high catalytic activity (33). It is
Similarly, for the tubular osmium-gold clus- also known that bimetallic catalysts often exhibit
ter [Osin(CO)?,(AuPPh,Me),](Figure 9) each superior operating stability to monometal sysgold atom can be assigned an oxidation state tems, and experiments have shown that rheniumof + 1 (29). Cyclic voltammetry studies show platinum clusters, supported on alumina, are
that this cluster undergoes two reversible one- effective catalysts for naphtha reforming (34).
electron reductions, indicating that there is However in these experiments the exact nature
no major change in core geometry with the of the cluster precursors and the catalytically
uptake of two electrons (27). This is consistent active materials were not known.
Recently, Shapley and co-workers have prewith the cluster being able to act as an “electron sink” since it is able to reversibly take up pared a set of supported bimetallic catalysts from
and release a pair of electrons repeatedly, with- two structural isomers of [RejIrC(C0)2,]’ by
out it degrading, or without any major struc- deposition onto high surface area alumina and
tural change that would cause the redox process activation in dihydrogen at 773 K (35). The specific activities of the catalysts depend on both
to become irreversible.
The situation for the osmium-mercury sys- the metal framework structure and the countems is more complicated. As noted earlier, all terion present in the precursor (either [NEt,]’
Platinum Metals Rev., 1998, 42, (4)
154
or [N(PPh,)2]+).Interpretation of EXAFS data
has enabled specific models to be developed for
the catalyst particle nanostructures which correlate with their catalytic activities. The more
active catalysts are modelled by a hemisphere
of close packed metal atoms, with an average
diameter of 1 nm, with iridium at the core. In
a series of related studies, Shapley has also shown
that [PtRu,C(CO),,] can be used as a neutral
cluster precursor for the formation of carbonsupported platinum-ruthenium nanoparticles
with exceptionally narrow size and composition
distributions (36). The bimetallic particles are
obtained by reduction of the carbido cluster with
hydrogen. A detailed structural model of the
nanoparticles was deduced on the basis of in situ
EXAFS, scanning transmission electron
microscopy, microprobe energy-dispersive Xray analysis and electron microdifiaction studies. These experiments show that the nanoparticles have a Ru:Pt ratio of 5:1, an average
diameter of approximately 1.5 nm and adopt
a face centred cubic close packed structure. This
is in contrast to the stable phase of the bulk alloy
which is hexagonal close packed. The EXAFS
studies also show that there is a non-statistical
distribution of different metal atoms in the
nanoparticles: the platinum atoms exhibit preferential migration to the surface of the particles
under an atmosphere of dihydrogen.
Of particular interest is a recent report by
Johnson, Thomas and colleagues that a cluster anion [ A ~ , R u , ~ C ~ ( C O ) , , C
(Figure
~ ] ~ - 16)
with a structure in which a central Ag, triangle
links two Ru, square based pyramids, closely
related to the copper-ruthenium systems
described above, had been successfullyanchored
inside mesoporous silica (37). Activation and
anchoring of the adsorbed cluster on the MCM41 silica support was achieved by heating the
sample under dynamic vacuum. EXAFS spectroscopy confirmed the presence of a bimetallic particle anchored to the silica oxygen atoms
through the silver atoms of the cluster. Highresolution electron microscopy of the heattreated material shows a uniform distribution
of the bimetallic nanoparticles aligned along the
zeolite channels. The catalytic performance of
the activated, supported bimetallic particles
was tested for hydrogenation of hex-1-ene to
hexane. Initial experiments showed a high selectivity (in excess of 99 per cent) and a turnover
frequency of at least 6300 mol hexane per mol
[Ag,RuIo]per hour.
T h e success of these studies led to the
incorporation of the copper-ruthenium cluster
described preanion, [ {RU,C(CO),,}~CU~CI.']~~,
viously (Figure 14) (32), into the mesoporous
channels of silica (38). Gentle thermolysis of
the anchored clusters gives the bimetallic
nanoparticles, characterised by X-ray absorption and FT-IR spectroscopies, and high-resolution scanning transmission electron microscopy. The copper and ruthenium K-edge X-ray
absorption spectra show that these catalytically
active particles have diameters of approximately
1.5 nm and display a rosette-shaped structure
with 12 exposed ruthenium atoms that are connected to a square base composed of relatively
concealed copper atoms. In turn, these are
anchored by four oxygen bridges to four silicon
atoms of the mesopore. The nanoparticles are
Fig. 16 The structure of the
metal core of dianion:
[A~J~&(CO)UC~]*-
Platinum Metals Rev., 1998, 42, (4)
155
active catalysts for the hydrogenation of hex1-ene, diphenylacetylene, phenylacetylene, stilbene, cis-cyclooctene and D-limonene, with
turnover frequencies of 22400, 17610,70, 150
and 360, respectively, at 373 K and 65 bar of
dihydrogen. The catalysts showed no tendency
to sinter, aggregate of fragment into their
component metals during these experiments.
Conclusions
In this review it has been shown that synthetic
strategies to prepare high nuclearity, bimetallic
clusters in good yields have been developed.
The metal cores of these clusters have dimensions in excess of 1 nm. By careful control of
reaction conditions it is possible to obtain specific target molecules with known ratios of the
two metallic components, and the methodology may be extended further to encompass the
majority of the late transition elements. In the
“condensed” clusters obtained, for the major-
ity of the osmiudmercury, rutheniudmercury,
osmiumigold and copperiruthenium systems
investigated, the mercury, gold or copper atoms
form a central domain and the osmium or ruthenium cluster units are fused onto the periphery
of these central units. In no case did the two
metallic components become dispersed throughout the metal core. Lastly, evidence is beginning to emerge that nanoparticles derived from
these and related clusters may prove to be active
catalysts when anchored on silica or alumina
supports.
Acknowledgements
My grateful thanks go to Professor the Lord Lewis
and Professor Brian F. G. Johnson for their support
and encouragement over the years, and for initiating
the research described in this review. I am also indebted
to the many research workers in the Department of
Chemistry, at Cambridge, who have carried out the
synthetic and structural work described, and to
Johnson Matthey for the generous loan of the heavy
transition metal salts.
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Construction of Miniature Organo-Rhodium Boxes
I
Many types of molecular cages exist in which
ions, atoms or molecules can be trapped. These
cages are usually held in suspension and are typically constructed from bifunctional ligands, with
square planar or tetrahedral metal centres at the
vertices. Until now there have been no cubic
shaped organometallic cages. However, if octahedral transition metal building blocks could
be constructed, then the assembly of cubicshaped structures should be possible.
Now, researchers fi-om the University of Illinois
have succeeded in constructing a molecular box
from a cubic array of cyano-linked rhodium and
cobalt octahedra (K. K. Klausmeyer, T. B.
Rauchfuss and S. R. Wilson, Angew. Chem.
Znt. Ed., 1998, 37, (12), 1694-1696).
Tricyanometalates Et4N[Cp’Rh(CN),] and
K[CpCo(CN),] (where Cp‘ = C5Me5,Cp =
Platinum Metals Rev., 1998, 42, (4)
1
C5H5)were used to prepare a series of molecular “squares”, by reaction with [Cp’RhCl,] or
[(cymene)RuCl,] (cymene = 4-isopropyltoluene). To assemble the box from the
“squares” the chloride ligands were removed by
AgPF,. The “molecular boxes” of most interest have the structure [(C5R5)8M8(p-CN),z]
(M
= Rh or Co) and are a subunit of hexacyanometalates, of which Prussian blue is one
example.
The most interestingbox has alternate rhodium
and cobalt atoms at the vertices, linked by CN
groups. Each metal atom can adopt its preferred
octahedral position. The box has edges 5.1 A
long with a volume of 132 A’, giving enough
space inside to encapsulate a caesium atom. The
box is also soluble so it could therefore be used
for trapping molecules in solution.
-
157